Polymers have served essential needs in society. For many years, these needs were filled by natural polymers. More recently, synthetic polymers have played an increasingly greater role, particularly since the beginning of the 20th century. Especially useful polymers are those prepared by an addition polymerization mechanism, i.e., free radical chain polymerization of unsaturated monomers, and include, by way of example only, coatings and adhesives. In fact, the majority of commercially significant processes are based on free-radical chemistry. That is, chain polymerization is initiated by a reactive species, which often is a free radical. The source of the free radicals is termed an initiator or photoinitiator.
Improvements in free radical chain polymerization have focused both on (1) more reactive monomer and pre-polymer materials and (2) the photoinitiator. Whether a particular unsaturated monomer can be converted to a polymer requires structural, thermodynamic, and kinetic feasibility. Even when all three exist, kinetic feasibility is achieved in many cases only with a specific type of photoinitiator. Moreover, the photoinitiator can have a significant effect on reaction rate which, in turn, may determine the commercial success or failure of a particular polymerization process or product.
A free radical-generating photoinitiator may generate free radicals in several different ways. For example, the thermal, homolytic dissociation of an initiator typically directly yields two free radicals per initiator molecule. A photoinitiator, i.e., an initiator which absorbs light energy, may produce free radicals by one of three pathways:
(1) the photoinitiator undergoes excitation by energy absorption with subsequent decomposition into one or more radicals;
(2) the photoinitiator undergoes excitation and the excited species interacts with a second compound (by either energy transfer or a redox reaction) to form free radicals from the latter and/or former compound(s); or
(3) the photoinitiator undergoes an electron transfer to produce a radical cation and a radical anion.
While any free radical chain polymerization process should avoid the presence of species which may prematurely terminate the polymerization reaction, prior photoinitiators present special problems. For example, absorption of the light by the reaction medium may limit the amount of energy available for absorption by the photoinitiator. Also, the often competitive and complex kinetics involved may have an adverse effect on the reaction rate. Moreover, some commercially available radiation sources, such as medium and high-pressure mercury and xenon lamps, may emit over a wide wavelength range, thus producing individual emission bands of relatively low intensity. Many photoinitiators only absorb over a small portion of the emission spectra and, as a consequence, much of the lamps"" radiation remains unused. In addition, most known photoinitiators have only moderate xe2x80x9cquantum yieldsxe2x80x9d (generally less than 0.4) at these wavelengths, indicating that the conversion of light radiation to radical formation can be more efficient.
Many commercially available photoinitiators, including IRGACURE(copyright) 369, are presently used in ink compositions to accelerate ink drying in xe2x80x9cradiation-drying printing.xe2x80x9d As used herein, the term xe2x80x9cradiation-drying printingxe2x80x9d refers to any printing method which utilizes radiation as a drying means. Radiation-drying printing includes, for example, offset printing operations, such as on a Heidelberg press, flexographic printing, and flatbed printing. Commercially available photoinitiator systems have a number of shortcomings. First, most of the commercially available photoinitiator systems require a relatively large amount of photoinitiator in the ink composition to fully cureldry the ink composition. This leads to undesirable extractables within the ink composition. Second, most of the commercially available photoinitiator systems require a high-energy radiation source to induce photocuring. Moreover, even with the high-energy radiation source, often the cure results are unsatisfactory. Third, many commercially available photoinitiator systems are highly reactive to oxygen and must be used under a nitrogen blanket. Fourth, even with a large amount of photoinitiator and a high energy light source, the commercially available photoinitiator systems require a dry/cure time only accomplished by multiple passes, as many as 15 passes, under a light source, which significantly limits the output of a radiation-drying printing press.
In view of the above drawbacks of the prior art, a new class of energy-efficient photoinitiators were developed which are disclosed in U.S. Pat. No. 6,486,227 to Nohr et al., which is incorporated herein by reference in its entirety. In Nohr et al., zinc-complex photoinitiators are disclosed. The photoinitiators may be cured in air as well as a nitrogen atmosphere. Further, the photoinitiators disclosed in Nohr et al. have excellent photoreactivity characteristics making them well suited for use in the radiation-drying printing industry.
Indeed, the photoinitiators disclosed in U.S. Pat. No. 6,486,227 represent advances in the art of photoinitiators. The present invention is directed to further improvements in the same class of photoinitiators disclosed in Nohr et al. In particular, the present invention is directed to further improving the stability of zinc-complex photoinitiators.
The present invention is generally directed to zinc-complex photoinitiators that have improved stability in some applications. In one embodiment, the photoinitiators of the present invention have the following general formula: 
wherein Z each independently represents 
wherein R1, R2, R3 and R4 each independently represent hydrogen, an alkyl group having from one to six carbon atoms, an alkoxy group having from one to six carbon atoms, or a halogen-substituted alkyl group; R5, R6, R7 and R8 each independently represent an alkyl group having from one to six carbon atoms, an aryl group, or a halogen-substituted alkyl group having from one to six carbon atoms; wherein X represents (R17)2O or (R17)3N, wherein R17 represents H or an alkyl group having from one to eight carbon atoms; and wherein R9, R10, R11 and R12 comprise an alkyl group, an aryl group, a halo group, an alkoxy group or hydrogen and wherein at least one of R9, R10, R11 and R12 comprises an alkyl, an aryl, a halo, or an alkoxy group.
For many applications, at least one of R9 or R10 and at least one of R11 or R12 above comprise an alkyl, an aryl, a halo, or an alkoxy group. By selecting particular xe2x80x9cRxe2x80x9d groups, photoinitiators are produced having a desired absorption maximum, which substantially corresponds to an emission band of a radiation source and selectively varies from less than about 290 nm to greater than about 350 nm. It has also been discovered that selecting particular xe2x80x9cRxe2x80x9d groups can further serve to increase the stability of the photoinitiators.
In another embodiment of the present invention, the photoinitiators of the present invention can have the following formula: 
wherein Y independently represents O, S, or Oxe2x95x90C; wherein R1, R2, R3 and R4 each independently represent hydrogen, an alkyl group having from one to six carbon atoms, an alkoxy group having from one to six carbon atoms, or a halogen-substituted alkyl group; R5, R6, R7 and R8 each independently represent an alkyl group having from one to six carbon atoms, an aryl group, or a halogen-substituted alkyl group having from one to six carbon atoms; wherein X represents (R17)2O or (R17)3N, wherein R17 represents H or an alkyl group having from one to eight carbon atoms; and wherein R9, R10, R11, R12, R13, R13xe2x80x2, R14, R14xe2x80x2, R15, R15xe2x80x2, R16, and R16xe2x80x2 comprise an alkyl group, an aryl group, a halo group, an alkoxy group, or hydrogen; R13xe2x80x2 R14xe2x80x2 R15xe2x80x2 and R16xe2x80x2 being the same or different from R13, R14, R15 and R16; and wherein at least one of R13, R14, R15 and R16 comprises an alkyl, an aryl, a halo, or an alkoxy group.
In the above photoinitiator, for many applications, at least one of R13 or R14 and at least one of R15 or R16 comprises an alkyl, an aryl, a halo, or an alkoxy group. In this embodiment, at least one of R9 or R10 and at least one of R11 or R12 may also comprise an alkyl, an aryl, a halo, or an alkoxy group.
The present invention is directed to the above-described photoinitiators, compositions containing the same, and methods for generating a reactive species which includes providing one or more of the photoinitiators and irradiating the one or more photoinitiators. One of the main advantages of the photoinitiators of the present invention is that they efficiently generate one or more reactive species under extremely low energy lamps, such as excimer lamps and mercury lamps, as compared to prior art photoinitiators. The photoinitiators of the present invention also efficiently generate one or more reactive species in air or in a nitrogen atmosphere. Unlike many prior photoinitiators, the photoinitiators of the present invention are not sensitive to oxygen.
The present invention is further directed to a method of efficiently generating a reactive species by matching a photoinitiator having an absorption maximum to an emission band of a radiation source, which corresponds to the absorption maximum. By adjusting the substituents of the photoinitiator, one can shift the absorption maximum of the photoinitiator from less than about 290 nm to greater than about 350 nm.
The present invention is also directed to methods of using the above-described photoinitiators to polymerize and/or photocure a polymerizable material. The photoinitiators of the present invention result in rapid curing times in comparison to the curing times of prior art photoinitiators, even with relatively low output lamps. The present invention includes a method of polymerizing a polymerizable material by exposing the polymerizable material to radiation in the presence of the efficacious wavelength specific photoinitiator composition described above. When an unsaturated oligomerimonomer mixture is employed, curing is accomplished.
The present invention further includes a film and a method for producing a film, by drawing an admixture of polymerizable material and one or more photoinitiators of the present invention, into a film and irradiating the film with an amount of radiation sufficient to polymerize the composition. The admixture may be drawn into a film on a nonwoven web or on a fiber, thereby providing a polymer-coated nonwoven web or fiber, and a method for producing the same.
The present invention is also directed to an adhesive composition comprising a polymerizable material admixed with one or more photoinitiators of the present invention. Similarly, the present invention includes a laminated structure comprising at least two layers bonded together with the above-described adhesive composition, in which at least one layer is a nonwoven web or film. Accordingly, the present invention provides a method of laminating a structure wherein a structure having at least two layers with the abovedescribed adhesive composition between the layers is irradiated to polymerize the adhesive composition.
The present invention is further directed to a method of printing, wherein the method comprises incorporating one or more photoinitiators of the present invention into an ink composition; printing the ink onto a substrate; and drying the ink with a source of radiation.
These and other features and advantages of the present invention will become apparent after a review of the following detailed description of the disclosed embodiments and the appended claims.